1. Field of the invention
The present invention relates to an electro-photographic image forming apparatus such as a printer, a copying machine, a facsimile and the like for electrostatically forming an image.
2. Related Background Art
The start up time period of the image forming apparatus such as the printer, the copying machine, the facsimile and the like greatly depends on the temperature of the fixing device. That is, in these image forming apparatuses, the start up time period from when the power is turned on to when starting up to the image formable state is greatly influenced by the time it takes for the fixing device to reach the temperature capable of performing fixation.
In order to reduce the start up time period, in recent years, the fixing roller is made thinner or an induction heating fixing method and the like is adopted. In the latter induction heating fixing method, the heat is generated at the fixing roller and the like by the eddy current produced by the magnetic field generated by the current flowing through the coil.
Consequently, the influence of the fixing device on the start up time period is reduced. On the contrary, there is increased the influence of factors, other than the fixing device, on the start up time period of the image forming apparatus, for example, the start up time period of an auxiliary device such as a feeder for reading the document or a finisher for sorting the output paper, or a potential control time of the photosensitive surface for outputting an appropriate image and the like.
This will be explained with reference to FIG. 12. In the figure, the axis of abscissa indicates time and the axis of ordinate indicates electric power. As shown in the figure, if the start up time period (warm up time period) of the fixing device is long, the start up time period of the auxiliary device and the potential control time are within the start up time period of the fixing device, and thus the time required until the image formable state is not greatly influenced. However, if the start up time period of the fixing device is reduced as shown in FIG. 13, the time required until the image formable state is greatly influenced by the start up time period of the auxiliary device and the potential control time. In order to reduce the influence of the time for potential control, a method of omitting the step for potential control to reduce the time period is disclosed in Japanese Patent Application Laid-Open No. 2000-181196, as shown in FIG. 14.
However, if the step for potential control is uniformly reduced, the problem in that the image stability may lack depending on the image forming operating environment arises.
Further, depending on the image forming operating environment, the time until image formable state may differ. For instance, under a low temperature environment in which the start up time period of the fixing device is significantly longer than the normal environment, or under a high temperature high humidity environment that requires a drum rotating time for removing the moisture adhered, due to high humidity, to the photosensitive member (photosensitive drum) with a drum heater, the time until transitioning to the image formable state becomes longer than normal due to factors other than the potential control.
Thus, depending on the image forming operating environment, the adjustment items for performing image formation is made uniform irrespective of the difference in time for transitioning to the image formable state, and when performing the remaining adjusting items after transitioning to the image formable state, the following problems arise. The start up time period becomes longer due to other factors, and even if there is extra time to perform the remaining adjusting items, the time for transitioning to the image formable state takes a long time, and further, the down time caused by implementation of the remaining adjusting items occur even if transitioned to the image formable state, thereby lowering the usability.